The IEEE standard 802.11a supplement to the IEEE standard 802.11 specification defines how to implement an orthogonal frequency division multiplexing (OFDM) system in the 5.25-5.35, and 5.450-5.705 GHz unlicensed national information structure bands As it happens, these frequency bands are shared by certain radar systems and so it is necessary, and required by the Federal Communications Commission, that all wireless communication devices utilizing these bands be able to sense the presence of a radar signal on a particular channel or channels and then quickly stop transmitting on the channel(s). Specifically, in a wireless LAN that includes a number of access points (APs) that transmit frames to and receive frames from wireless communications devices, it is required that master devices, which could be the APs for instance, are all capable of detecting the presence of radar signals in the 5 GHz frequency spectrum and notifying all of the slave devices, which in this case could be the wireless communications devices, that radar is present in one or more channels and that the slave devices should stop transmitting on those channels. More specifically, prior to establishing a session with a slave wireless communications device, the master devices are required to sense the medium for some minimum, specified time period to determine whether radar signals are present in any particular channel(s). If no radar signal is detected in a particular channel, it is said that this channel is “available” to be utilized by the wireless communications device to establish a communications session. As mentioned above, if a master device detects a radar signal in any particular channel during the minimum, specified period of time, this channel is marked as not available. Furthermore, a slave device is not permitted to transmit frames prior to receiving a signal from a master device indicating that a channel is available for use. This signal could be any qualified signal transmitted by the master device such as a beacon or a probe response for instance.
The protocol described above is generally referred to as dynamic frequency selection (DFS) and it operates in the background of a wireless LAN so as to be largely transparent to the user of a wireless communications device. However, the continuity of a communications session between an AP and an associated wireless communications device can be interrupted in the event that the user of the wireless device moves around in space. Such movement is generally referred to as roaming and it results in the wireless communications devices having to disassociate with a current AP, whose transmission signal quality is deteriorating, and associate with another AP whose signal quality is better than that of the AP it is currently associated with.
The handoff process whereby a wireless communications device becomes disassociated from an AP it is currently communicating with and associates with another AP can be managed by either the AP or by the wireless communications device. Assuming that the process is managed by the wireless communications device, it monitors the channel signal quality with the AP it is currently associated and also monitors the signal quality of a signal or signals it receives on a different channel or channels from one or more other APs that are within transmission range and with which it is not currently associated. The handoff process can be initiated at the point in time that the signal quality of the current communications link falls below a particular threshold. In order for a wireless communications device to associate with a new AP, it is necessary to first conduct an authentication process and then an association process. At the point that the wireless communications device sends an association request to a new AP, the current AP with which the wireless device has a communications link is free to stop sending frames to the wireless device. At this point, it is essential that the new AP respond very quickly to the wireless communication devices request to associate and that the new communications link is established with a minimum of delay. In practice there are delays, but such delays are not typically noticeable or objectionable to the user. The threshold mentioned above with reference to signal quality can be a signal strength value as measured in decibels or a packet error rate value as measured in bits per second, for instance or the threshold can be the result of a comparison of the current signal quality with a candidate signal quality stored in memory where the threshold is reached a the point the current signal quality falls below a candidate signal quality.
Continuing to refer to the handoff process in more detail, a wireless communications device typically transmits probes on one or more channels in order to receive probe responses from other APs with which it might associate that will permit a higher quality communications link. The operation of transmitting a probe and receiving a probe response for each channel typically occurs between periods of transmission and reception of voice or other packets to or from the AP with which the wireless device is currently associated. Unfortunately, sending such probes in the 802.11a environment is not allowed prior to the wireless device knowing that the channel on which it is sending the probe is available, or radar free. The FCC currently specifies that master devices in the 5 Mhz spectrum must sense the wireless medium for sixty seconds on a channel shared with radar before the device transmits over that channel. This means that either the wireless communications device is responsible to scan each channel for sixty seconds before the handoff process can begin or that the AP is responsible for scanning the medium for available channels. Clearly, it is not practical for the wireless communications device to scan the medium for such a period of time, as this would cause an unacceptable delay in handing off a communications session from one access point to another during the roaming process.
Another problem arises if a slave device is off channel communicating with an AP other than the one it is currently associated. If, during the time that the slave device is off channel, the AP with which it is currently associated transmits a change channel signal, the slave device would not detect this signal and would continue to transmit on the channel in possible conflict with a radar signal.
Generally, an AP in a wireless LAN operating according to the standard 802.11 specification transmits two types of signals that contain all of the information that a wireless communications device needs in order to complete the hand off process. One is a management type frame called a beacon and the other is a control type frame called a probe response and they are collectively referred to as qualified transmissions. As it is necessary for a wireless communications device to transmit a probe signal to an AP in order to receive a probe response from the AP, and as such activity is not permitted in channels shared with radar, it is necessary to implement some method whereby the wireless communications device can rely on a qualified signal, such as a beacon, that can be received without having to actively request it. On the other hand, beacons are only transmitted by AP's infrequently, every 102.4 msec for instance. Consequently, it is necessary for the wireless device to passively scan the medium for some period of time, during periods not otherwise employed by the communications device to transmit or receive frames from the AP with which it is currently associated, to detect a beacon. In other words, the wireless communications device operates in a time-sliced manner to communicate with the AP with which it is currently associated and to scan all of the other channels, both those shared with radar and those not shared with radar, in order to receive a signal indicating that some other channels are available on which to communicate with another AP.
Furthermore, it is important from the perspective of the wireless communications device user that the communications session is of consistently high quality. That is, the audio quality is of high fidelity, that the transmitted or received frames of audio information are not clipped or dropped and that there is no noticeable or only minimal delay added as the result of the hand off process.
In light of the above problems associated with the hand off procedure in a network that shares at least a portion of its frequency band with radar signals, it is desirable that the hand off of a wireless communications device from one AP to another will not result in any noticeable discontinuity in the communication session. Further, it is desirable that the amount of time it takes to complete the hand off of a wireless communications device session from one AP to another AP is predictably and suitably small. Still further, it is required that the slave devices are able to detect a signal from the AP, with which they are associated, to stop transmitting on channels that the master device marks as unavailable.